[Block: Image] Allow overlay gradient

[luminuu]

What problem does this address?

Currently, the Cover block and newly the Post Featured Image block allow gradient overlays to apply further styling to the image. The Image block however only supports duotones at the moment.

In my style variation I designed for Twenty Twenty-Three, I added default gradients to the images to create a unique design:

Link to Twenty Twenty-Three repo: https://github.com/WordPress/twentytwentythree

The top image with the blue-green gradient is an Image block in the template files. Below, Post Featured Images are used in the Query block.

What is your proposed solution?

To enable similar styling options across all blocks displaying images, the Image block should allow overlay gradients as well. It has been recently added to the Post Featured Image block: #43838

In order to make this work for themes and style variations, the overlay functionality should also be added to theme.json to be configured there too. It's currently not available at all, but there's already an issue: #27809